What a mistake! I got so many shots that were out of focus, such as this one (16-50 PZ at f/5.0):
So I downloaded a focus test chart from Jeffrey Friedl and investigated into the focus issue. Turned out, the camera always do back focus while using AF-A or AF-C modes. I tested with a6000 + SEL50F18 and a5100 + SEL35F18 and both behaved exactly the same.
This back focus issue is easier to notice at f/2.2 and f/2.5, and not so noticeable at f/2.0 and f/1.8. It appears to be more severe in AF-C mode than AF-A. Above photo was taken in AF-A mode.
Once I switch to AF-S, focusing becomes normal:
Although I only present two test charts here, I've done lots of tests to reach the pattern which is totally repeatable. Hopefully Sony will address this issue with a new firmware update. Until then, we'd better avoid AF-A and AF-C and live with AF-S.
[update] I did more tests, now I can safely say:
- For non-moving objects, AF-C is not as accurate, dependable and consistent as AF-S. This is normal, because AF-C is ought for tracking moving objects.
- AF-A may use either AF-S or AF-C algorithm for non-moving objects. This is not right, because AF-A should always be identical to AF-S, not AF-C, if everything is still.
The following photo and video show that AF-A picked improperly AF-C over AF-S: